In the coating of a web of paper or board, the surface of the web is first coated with a furnish containing coating pigments slurried in water. After the application and smoothing of the coating mix, the coating applied to the web surface as well as the underlying base web must be dried to a sufficiently low moisture for final use or further processing. Hence, a major portion of the energy consumed in the production of coated paper grades is lost in drying the web during the different steps of postprocessing, which means that energy management in drying is an extremely vital factor contributing to the profitability of production. Correct drying technique also affects the quality of the produced paper grade. Another parameter highly pertinent to the quality of produced paper is the control of the machine-direction moisture profile, that is, the moisture of the base paper, which must be kept at a constant level during the run. The web moisture content affects particularly the paper web behavior in calendering and printing. As modern production lines are equipped with on-line calendering, wherein the coated web is passed directly to a calender, the moisture profile of the running web has an insufficient time to reach a uniform equilibrium state prior to calendering, a situation which is in contrast to that attainable in the traditional off-line calendering, wherein the coated web was stored in a machine reel prior to subsequent calendering. Correspondingly, the transport chain of paper from the mill to printing houses and other users has been speeded up, whereby the moisture even in uncalendered paper does not necessarily have enough time to stabilize and reach a sufficiently low level prior to printing. In coating, the web moisture content affects the penetration of water into the base web during the application of the coating mix and, as a result, the change of coating solids content after coating. As variations in the solids content of the coating are reflected in plural parameters in the application process, it is important to keep the web moisture during application and drying accurately within proper limit values in order to attain a uniform and desired final quality of the product.
Conventionally, a coated web is dried immediately after the application of coating using noncontacting dryers, which step may be followed when necessary by cylinder dryers and other dryers of the contacting type. The moisture content of the running web is measured at multiple points along the web travel in the coater apparatus and, on the basis of the measurement data, the drying effect of each dryer is individually adjusted so as to attain a proper web moisture over the cross-machine width at the respective measurement point as well as an average moisture content that stays between given limits during a run, the latter requirement meaning that the machine-direction moisture profile is controlled to a given set value. The overall drying capacity is adjusted to a suitable basic level based on test runs and data accumulated from a long-term experience in the art, and the individual dryer effects are then fine-tuned during the run on the basis of measurement data either automatically or manually. Conventionally, one of the dryers or one dryer group is selected to perform as the controller of the final moisture level, whereby the heating power input to the selected dryer group(s) is adjusted by means of a feedback signal obtained from the measurement system. In this arrangement, the other dryers are driven under manual control. Such a control scheme responds very tardy and compensation for the slow response of dryer control is difficult to implement in situations requiring a fast change of dryer effect levels. Furthermore, the web temperature prior to the coater apparatus must be kept sufficiently low to avoid floccing of the coating mix being applied. Hence, proper control of the drying effect is important particularly in the final stage of the dryer section prior to the subsequent coating step. The web temperature also affects the final quality of the coated web.
Particularly in situations of changing running conditions or when starting up the machine, known in the art as the run-up, the elevation of the dryer drying effect levels to correct values and adjustment of the same to proper run-time levels requires excellent skills from the personnel operating the machine. However, carrying out the procedure of setting the dryer evaporation effect levels in the coater section to correct values under run-up or changing process conditions takes time, during which the produced paper or board falls short of the specified quality requirements thus necessitating dumping of the web into the pulper. Hence, it is advantageous to minimize the durations of run-up and process value change times in order to achieve improved production efficiency at the machine. The above control scheme is also extremely clumsy in the optimization of drying energy consumption inasmuch it relies on the control of each dryer unit separately, whereby the mutual evaporation effect ratios between the dryer units are difficult to alter in an uncomplicated manner. Furthermore, a failure in one or a greater number of the dryer units is difficult to compensate for, because the process is designed for operation with all the dryer units being functional.